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Re: [Discuss-gnuradio] Question about LFTX and RFTX

From: Brian Padalino
Subject: Re: [Discuss-gnuradio] Question about LFTX and RFTX
Date: Fri, 31 Aug 2007 00:37:10 -0400

On 8/30/07, Ismail Mohamed <address@hidden> wrote:
> Hi again,
> Recently there has been talk about the I and Q components of the
> signal.  I understand the analytic signal concept, also the
> implementation as shown on:
> http://gnuradio.org/trac/wiki/UsrpRfxDiagrams
> where it says,
> "Right idea, except that there is effectively a single complex
> multipler. I.e., I2+jQ2 = (I1+jQ1) * exp(jwt)."
> So the analytic signal is generated on the motherboard and mixed up to a
> higher carrier on the daughterboard, i.e. to a new 'w' but same I and Q.
> My question is that, with the LFTX and LFRX, if I tune the daughterboard
> to DC, this would make w=0.  So there would be no carrier.  How are the
> I and Q components separated?  Or is it simply that each alternate
> sample is I and Q, and in essence we are sending a purely real signal,
> but saying each second sample represents the imaginary?

>From my limited DSP understanding, at baseband, I and Q are simply
phase relationships and amplitude information around and within the
unit circle.

With that being said, I don't think I and Q can be combined into a
single real signal while preserving information simply because DC has
no phase component.

On the other hand, if the LF cards have both I and Q inputs/outputs,
then you should be able to see essentially two signals come out and
maintain their proper information.  If it's only real - you'll
probably have to mix up to some IF to maintain all information.

I believe the real question here is if you can do complex sampling on
a real signal centered strictly around DC - which I don't think you

Anyone wish to comment?  I am most likely wrong, but I'd definitely
like to know the real answer to this as well.


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